Supercapacitors and other electrodes and methods for making and using same

ABSTRACT

Systems and methods involving nanomaterial-based electrodes, such as supercapacitor and battery electrodes that can be flexible, are described.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 62/852,124, filed May 23, 2019,and entitled “SUPERCAPACITORS AND OTHER ELECTRODES AND METHOD FOR MAKINGAND USING SAME” and U.S. Provisional Patent Application Ser. No.62/849,458, filed May 17, 2019, and entitled “ULTRAHIGH AREALCAPACITANCE FLEXIBLE SUPERCAPACITOR ELECTRODES ENABLED BY CONFORMAL P3MTON HORIZONTALLY-ALIGNED CARBON NANOTUBE ARRAYS,” each of which isincorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

Systems and methods involving nanomaterial-based electrodes, such assupercapacitor and battery electrodes that can be flexible.

BACKGROUND

The increasing demand for portable and flexible electronics such asroll-up displays, bendable transistors, electronic papers, and wearablesensing devices, has inspired the development of flexible energy storagedevices. Supercapacitors have been explored in this arena.

Some of these electrodes include nanotubes without significant order intheir arrangement. Others include some degree or order, such asnanotubes oriented perpendicular to electro surfaces, conductive fabricsarranged in a weave or approximate weave, or the like. Results have beenmoderate. Nanocarbon electronic conductors combined withpseudocapacitive material, such as conducting polymer or metal oxides,typically have involved arrangements (geometrical and/or material) thathave inhibited potential ion transport and capacitance, particularlyunder high current density necessary for devices requiring high powerdensity and charge/discharge rates.

SUMMARY

The present invention involves arrangement of nanomaterials at surfacesproviding advantageous electronic transport phenomena associated withthe surfaces, which can form parts of electronic devices such assupercapacitors, batteries, etc. The nanomaterials can be associatedwith an auxiliary pseudocapacitive material, for example coated with apolymer of appropriate electronic conductivity, to affect the materials'electronic properties and thereby the overall properties of theelectrode or other device.

Methods of making and using such arrangements and devices are alsoprovided.

In one aspect, articles are described. In some embodiments, the articlehas a surface, and comprises electronically-conductive nanostructures, amajority of which include a longest dimension oriented substantiallyparallel to the surface and oriented more greatly in a first directionparallel to the surface than in another direction perpendicular to thefirst direction, wherein a majority of the electronically-conductivenanostructures are associated with a pseudocapacitive material.

In certain embodiments, the article has a surface and compriseselectronically-conductive nanostructures, at least some of which includea longest dimension oriented substantially parallel to the surface, thearticle made by the process of growing nanostructures at the surface inan orientation substantially perpendicular to the surface, thenre-arranging a majority of the nanostructures such that they becomeoriented with a longest dimension substantially parallel to the surface.

In some embodiments, the article comprises electronically-conductivenanostructures, and a material comprising poly(3-methylthiophene)conformally coated over a majority of the electronically-conductivenanostructures.

Some aspects are related to methods. In some embodiments, the methodcomprises associating pseudocapacitive material with a plurality ofelectronically-conductive nanostructures.

The subject matter of the present invention involves, in some cases,interrelated products, alternative solutions to a particular problem,and/or a plurality of different uses of one or more systems and/orarticles.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale unless otherwiseindicated. In the figures, each identical or nearly identical componentillustrated is typically represented by a single numeral. For purposesof clarity, not every component is labeled in every figure, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention.

FIG. 1A is a cross-sectional schematic diagram of an article comprisingnanostructures associated with pseudocapacitive material, in accordancewith certain embodiments.

FIG. 1B is a schematic illustration, according to certain embodiments,of a plurality of nanostructures.

FIG. 2 is a perspective view schematic illustration of a devicecomprising nanostructures arranged in patterns, according to certainembodiments.

FIGS. 3A-3D are, according to some embodiments, illustrations and photoimages of electrodes based on horizontally-aligned CNTs (HACNTs) withpseudocapacitance via conformal P3MT: FIG. 3A shows the fabricationprocess of a P3MT/HACNT composite electrode with illustrations of thecharged status of HACNT and P3MT/HACNT positive electrodes. FIG. 3B isthe chemical structure of P3MT.

FIGS. 3C and 3D are optical images of composite electrodes with flexible(FIG. 3C) and rollable (FIG. 3D) properties at a radius of curvature of5 mm.

FIGS. 4A-4D show, according to certain embodiments, properties of aP3MT/HACNT nanocomposite and HACNTs: FIGS. 4A-4B show SEM images ofHACNTs (FIG. 4A) and a P3MT/CNT (FIG. 4B) composite. FIG. 4C depicts theTEM image of a P3MT/CNT composite with coating thickness of ˜5 nm. FIG.4D shows Raman spectra of a P3MT/CNT composite. D-band and G-band arefrom HACNTs, and the band at 1430 cm⁻¹ is from P3MT.

FIGS. 5A-5B are, according to certain embodiments, plots showingelectrochemical performance of single electrode comprised of aP3MT/HACNTs composite, HACNT film, and buckypaper: FIG. 5A shows cyclicvoltammetry curves at 100 mV s⁻¹, and FIG. 5B shows areal capacitancecomparison of the three electrodes.

FIGS. 6A-6E illustrate, according to some embodiments, electrochemicalperformance of assembled asymmetric supercapacitor cells based onP3MT/HACNT and HACNT electrodes. FIG. 6A is a schematic illustration ofan asymmetric supercapacitor. FIG. 6B is a plot showing CV curves ofcells at various scan rates. FIG. 6C is a plot showing galvanostaticcharge/discharge curves of cells at various current densities. FIG. 6Dis a plot showing the areal capacitance of cells at different currentdensities. FIG. 6E is a Ragone plot of cells and the comparison withother works.

FIGS. 7A-7D illustrate, according to certain embodiments, bending testsof asymmetric cells. FIG. 7A illustrates flat and bending (folded, withbending angle 180°, and radius of curvature ˜5 mm) states of cells. FIG.7B is a plot showing CV curve comparison at 100 mV s⁻¹ of flat andbending states. FIG. 7C is a Nyquist plot comparison of flat and bendingstates. FIG. 7D shows cycling test results with bending every 1000cycles.

FIG. 8 is, according to some embodiments, a scanning electron microscope(SEM) image of buckypaper with random-dispersed carbon nanotubes.

FIG. 9 is, according to certain embodiments, a plot showing GCD curvesof different electrodes at alternating current densities of 10 mA cm⁻²and −10 mA cm².

DETAILED DESCRIPTION

Systems and methods involving electrodes, which can form the basis ofelectronic devices, are described. The invention facilitates operationunder high power conditions and rapid charging, high power density andhigh energy density technologies.

One aspect of the invention involves the discovery of drawbacks in someelectrodes that form the state of the art, namely, electrodes thatinclude electronically conductive carbon nanotubes and/or otherstructures that have been recognized by the inventors as havinginsufficient order in their arrangement for adequate performance. Manyprior electrodes include less-ordered, even randomly-dispersedconductive nanotubes in flexible substrates, leading to tortuous ionpathways inside the electrode, impeding ion transport. Other priorelectrodes include conductive materials arranged in weaves or the like.The recognition of drawbacks of some prior art arrangements is oneaspect of the invention. Other aspects are described below with respectto electrodes, capacitors, systems, and methods.

Certain embodiments relate to nanocarbon electronic conductors combinedwith pseudocapacitive materials, such as conducting polymers or metaloxides. These composites can display outstanding electrochemicalproperties and mechanical flexibility. These characteristics in thinfilm form, can allow for the fabrication of flexible and lightweightelectrodes for novel electrochemical energy storage devices likesupercapacitors.

In accordance with certain embodiments, flexible densifiedhorizontally-aligned CNT arrays (HACNTs) with controlled nanomorphologyfor advantaged ion transport paths are introduced havingconformally-coated poly(3-methylthiophene) (P3MT) conducting polymer toimpart pseudocapacitance. The resulting P3MT/HACNT nanocompositeelectrodes can exhibit high areal capacitance. For example, theresulting P3MT/HACNT nanocomposite electrodes can exhibit an arealcapacitance of 3.1 F cm⁻² at 5 mA cm⁻², with areal capacitance shown toremain high at 1.8 F cm⁻² even at a current density of 200 mA cm⁻². TheP3MT/HACNT positive electrode assembled in an asymmetric supercapacitorcell with HACNTs as the negative electrode can deliver more than 1-2orders of magnitude improvement in both areal energy and power densityover state-of-the-art all-nanocarbon cells (e.g., at 1.1 mWh cm⁻² and1.75 W cm⁻², respectively). Further, in accordance with certainembodiments, little change (<0.5%) in cell performance is observed underhigh strains due to bending (e.g., in a 5 mm radius of curvature undercyclic operation), demonstrating mechanical and electrochemicalstability of the hybrid nanostructured composite electrodes.

In one aspect, the invention involves an article having a surface, andincluding, among other possible components, electronically-conductivenanostructures, a majority of which include a longest dimension orientedsubstantially parallel to the surface, and oriented more greatly in afirst direction parallel to the surface than in another directionperpendicular to the first direction. It has been discovered that thisorientation, alone or in combination with certain other materialsassociated with the nanostructures, such as coatings or the like, andarrangements described herein, can provide significantly advantageouselectronic and ionic performance.

Where electronically-conductive nanostructures are orientedsubstantially parallel to the surface, they may also be orientedsubstantially non-parallel to the surface of the growth substrate (whenthey are grown on the substrate such as by chemical vapor deposition).In some cases, the longitudinal axes of the nanostructures are orientedin a substantially perpendicular direction with respect to the surfaceof the growth substrate. As described more fully below, an advantageousfeature of some embodiments of the invention may be that the alignmentof nanostructures in the nanostructure forest may be substantiallymaintained, even upon subsequent processing (e.g., application of aforce to the forest, transfer of the forest to other surfaces, and/orcombining the forests with secondary materials such as polymers, metals,ceramics, piezoelectric materials, piezomagnetic materials, carbon,and/or fluids, among other materials).

In this aspect, a majority of the nanostructures include a longestdimension oriented substantially parallel to the surface. One example ofsuch an arrangement is shown in FIG. 1A, which illustrates article 100in which nanostructures 110 include longest dimensions that are orientedsubstantially parallel to surface 103 of substrate 102. This can beaccomplished in a variety of ways and, in one set of embodiments, asillustrated in figures presented in the examples below, elongatedelectronically conductive nanostructures are presented at a surface in asubstantially vertical arrangement (oriented substantially perpendicularto the surface). This can be accomplished by, for example, growingcarbon nanotubes on a surface. Subsequently, the nanostructures can bere-arranged so as to be positioned with a majority having a long accessoriented substantially parallel to the surface. This can be accomplishedby rolling a mechanical device across substantially vertically-alignednanotubes, as described in the figures herein, or other operations whichare available to those of originally skill in the art and which could beroutinely implemented based on the teachings of this disclosure.

In one set of embodiments of facilitating this arrangement ofnanostructures, as shown in figures herein, long axes are both orientedsubstantially parallel to the surface and oriented substantiallyparallel to each other. This does not mean that all long axes of allnanostructures are oriented in this way, it means that a majority are sooriented, such that, upon visual inspection of, for example of an SEMimage, those of ordinary skill in the art will conclude that alignmentis more parallel to the surface than otherwise. In another set ofembodiments, not only is alignment more parallel to the surface thanotherwise but a majority of the nanostructures are aligned in one axisparallel to the surface than the axis perpendicular to the first axis(and also parallel to the surface). This particular set of embodiments,although not critical to the invention, is but one example and can bedescribed in contrast to a traditional weave in which approximately halfof the nanostructures would be aligned with one axis parallel to thesurface and another approximate half would be aligned with an axis alsoparallel to the surface but perpendicular to the first axis. While thatarrangement (a majority of nanostructure axes parallel to the surface,but not directionally aligned with each other within the plane of thesurface) is also within the scope of certain embodiments of theinvention, so long as this otherwise facilities good electronicproperties, in another set of embodiments, as noted, it is not.

In the arrangement described here and in connection with figures hereinin which substantially vertically-aligned nanostructures (for example,carbon nanotubes grown from a catalytically-facilitated chemical vapordeposition process) are re-oriented so as to be substantially parallelto the surface, in one set of embodiments most ends of thenanostructures that had been at the surface remain substantiallyattached to the surface while the opposing ends, bent parallel to thesurface, are not attached to the surface. In another set of embodimentsthe nanostructures, in the re-arrangement process, substantially detachfrom the surface so that the resulting arrangement includesnanostructures, a majority of which are substantially parallel tosurface, with a majority of ends not attached to the surface or in someembodiments where essentially no ends of nanostructures are attached tothe surface.

Without wishing to be bound by any theory, advantageous electronic,ionic, and mechanical performance of embodiments of the invention may befacilitated by nanostructures substantially aligned parallel to asurface such that junctures from one nanostructure to another are betterfacilitated and lead to better electronic transport.

In one set of embodiments a pseudocapacitive material is associated withsome or all of the nanostructures at the surface. As used herein,“pseudocapacitive material” means a material that can interact with thenanostructures in a way that balances electronic conductivity with theability to enhance capacitive function of the nanostructures. Forexample, a pseudocapacitive material can be co-mixed with nanostructuresthat are then applied to a surface, or nanostructures can be formed on asurface and then a pseudocapacitive material can be added, for examplevia vapor deposition, precipitation from solution, or other techniqueavailable to those of ordinary skill in the art. The pseudocapacitivematerial, in one set of embodiments, does not define a continuouscarrier phase within which nanostructures are embedded, but insteaddefines a phase associated with nanostructures, the overall arrangementbeing porous. The porous structure can be controlled by those ofordinary skill in the art and selected based on the teachings of thisdisclosure to allow sufficient access by the nanostructures to a medium,such as an electrolyte, providing the electronic and ionic connectivityand conductivity to all aspects of the device for appropriate function.For example, nanostructures can be conformally coated (a coatingconforming to enough of the surface of the nanostructure so as tofacilitate electronic transport) while spacing between coatednanostructures defines porosity sufficient for device function, such asion transport. One example is illustrated in inset 109 of FIG. 1A, inwhich nanostructures 110 are coated with a conformal coating ofpseudocapacitive material 106. This can be accomplished in a number ofways such as formation of nanostructures at the surface as describedand, before or after any rearrangement necessary to render the majorityaligned parallel to the surface (if begun with alignment not parallel tothe surface), and chemical vapor deposition of a conductive polymerprimarily at the surfaces of the nanostructures. In some embodiments, itcan be advantageous to coat the nanostructures after they have beenrearranged to render the majority aligned parallel to the surface (e.g.,by rolling over the nanostructures with a roller, or by any othersuitable method). The ability to conformally coat the nanostructuresafter they have been so rearranged was unexpected, and it is believedthat coatings produced in this manner can exhibit improved electronicand/or mechanical characteristics relative to coatings that are appliedprior to rearranging the nanostructures (e.g., due to a reduction orelimination of mechanical stresses imposed on the coating ofpseudocapacitive material during rearrangement of the nanostructures).

The pseudocapacitive material used in such coating or other arrangementshould be selected to be of sufficient electronic and/or ionicconductivity to facilitate electronic transport to and from thenanostructures and between nanostructures, while having propertiessufficient to facilitate charge storage associated with the coatednanostructures (charge storage either at the surface of thenanostructures and/or surface of the coated nanostructures).Pseudocapacitive material, in one set of embodiments, comprises aconductive polymer such as poly(3-methylthiophene), polyaniline, or thelike. Those of ordinary skill in the art can select such materials basedon the teachings herein. In one set of embodiments, a conductive polymeris selected, as a pseudocapacitive material, such that one unit ofcharge can be stored per monomer unit of polymer. The ease or difficultyof inserting or removing charge from the material, such as the polymer,should be considered by those of ordinary skill in the art in selectingsuch materials for use in this invention. In some embodiments, thepseudocapacitive material is or comprises an electronically and/orionically conductive polymer. In some embodiments, the pseudocapacitivematerial comprises one or more metal oxides.

Coatings of pseudocapacitive material can be at a variety of dimensions.In one set of embodiments, nanostructures are coated at an averagethickness of between 0.1 and 50 nm, or between 1 and 50 nm, or between 2and 50 nm, or between 3 and 25 nm, or between 3 and 10 nm. In someembodiments, nanostructures have a starting (uncoated) diameter of lessthan 1 micron (i.e., 1 micrometer, or 1000 nanometers), less than 500nm, less than 250 nm, or less than 100 nm. In one set of embodimentsnanostructures have a starting (uncoated) diameter of between 2 and 15nm or between 4 and 10 nm. Coated nanostructures can have a diameter,including nanostructure and coating, of between 5 and 50 nm, 7 and 30 nmor between 12 and 25 nm, on average, in one set of embodiments. In someembodiments, the pseudocapacitive material coats the nanostructures suchthat the nanostructures have an average, coated diameter of at least 0.1nm, or at least about 0.5 nm, or at least about 1.0 nm.

Nanostructures of the invention can be selected from a wide variety ofspecies as described elsewhere herein. As one example, carbon nanotubesare used, such as single-walled carbon nanotubes.

Substrates can be selected by those of ordinary skill in the art basedon a particular need. The substrate can be a current collector, or anydevice with an electronically conductive surface adjacent thenanostructures, and connected or connectable to a circuit, whenappropriate.

Volumetric loading of the nanostructures can be selected by those ofordinary skill in the art across a variety of ranges to facilitate goodoperation. In one set of embodiments the invention involves a packingdensity (volumetric presence) of nanostructures, coated or not, inexcess of 1%, 5%, 10%, 15%, or 20% or greater. In some embodiments, theinvention involves a packing density (volumetric presence) ofnanostructures, coated or not, of 78% or less, 65% or less, or 50% orless.

The scale of porosity, that is, the spacing between nanostructuresassociated with the surface, whether coated or not, in one set ofembodiments is within 10% of the dimension of the positive or negativeions utilized in an electrolyte used with the device. Ion diffusion inand out of the porous structure can thus be facilitated for use inembodiments requiring this. A variety of porosities as descried hereincan be used. In one set of particular embodiments, porosity of 5-15 nm(nanometers) in average dimension, and averaged across pores, isselected. In another set of embodiments porosity between 5 nm and 100 nmis provided. In some embodiments, at least 10%, at least 20%, or atleast 30% (and/or, less than or equal to 90%, less than or equal to 80%,or less than or equal to 70%) of the volume of an electrode in which thenanostructures are arranged is externally accessible to a fluid (e.g., aliquid electrolyte). In embodiments in which the nanostructures are partof an electrode that is part of a capacitor or other power-producingdevice, any of a variety of electrolytes can be used. In someembodiments, aqueous electrolytes can be used. In certain embodiments,non-aqueous (e.g., organic) electrolytes can be used.

In one set of embodiments devices are provided with flexible substrates,resulting in overall electrodes or complete devices that can beflexible. One aspect of the invention is the discovery that orientationof nanostructures as described herein on a flexible substrate allows thesubstrate to be flexed in an operational device with essentially zero orlow loss of performance. For example, substrate surfaces of theinvention can be reoriented from a first configuration, which can beessentially flat or can be curved, to a second configuration where theradius of curvature changed between the first orientation and the secondorientation by at least 5%, 10%, 15%, 20%, 40%, 50%, or more with lossof one or more performance parameters of less than 50%, less than 25%,less than 10%, less than 5%, or less than 2% in value.

In some embodiments, the electronically-conductive nanostructures arearranged in a pattern. Such patterns may be achieved, for example, byarranging catalyst on a substrate in a pattern and subsequently growingnanostructures from the patterned catalyst. In some embodiments, thearticle comprises a first set of electronically-conductivenanostructures arranged in a first pattern and a second set ofelectronically-conductive nanostructures arranged in a second patternthat is interdigitated with the first pattern. One example of such anarrangement is shown in FIG. 2 . In FIG. 2 , article 200 comprises afirst set of electronically-conductive nanostructures 201A arranged in afirst pattern and a second set of electronically-conductivenanostructures 201B arranged in a second pattern that is interdigitatedwith the first pattern. In FIG. 2 , nanostructures 201A are arrangedover current collector 202A (which may be formed, for example, from thecatalyst used to grow nanostructures 201A) which is in turn arrangedover substrate 102. Similarly, in FIG. 2 , nanostructures 201B arearranged over current collector 202B (which may be formed, for example,from the catalyst used to grow nanostructures 201B). Current collector202A and/or 202B can be arranged over substrate 102.

In some embodiments, the nanostructures can be arranged within anelectrode in a pattern having relatively small features (e.g.,milliscale, microscale, or even nanoscale features). In someembodiments, the pattern in which the nanostructures are arrangedcomprises at least one elongated feature having a cross-sectionaldimension, measured parallel to the plane on which the nanostructuresare arranged and perpendicular to the length of the elongated feature,of less than or equal to 1 centimeter, less than or equal to 100millimeters, less than or equal to 10 millimeters, less than or equal to1 millimeter, less than or equal to 100 microns, less than or equal to10 microns, less than or equal to 1 micron, or less than or equal to 100nm. In some embodiments, the pattern in which the nanostructures arearranged comprises at least one elongated feature having across-sectional dimension, measured parallel to the plane on which thenanostructures are arranged and perpendicular to the length of theelongated feature, of greater than or equal to 10 nm.

In some embodiments, the nanostructures are part of an electrodecomprising a plurality of elongated protrusions extending outward from abase structure. The electrode can be, in certain embodiments, one of apair of interdigitated electrodes. One example of this arrangement isshown in FIG. 2 , in which interdigitated electrodes 201A and 201B eachcomprise a plurality of elongated protrusions 210 extending from basestructure 212. In some embodiments, some or all of the elongatedprotrusions have a cross-sectional dimension, measured parallel to theplane on which the nanostructures are arranged and perpendicular to thelength of the elongated feature, of less than or equal to 1 centimeter,less than or equal to 100 millimeters, less than or equal to 10millimeters, less than or equal to 1 millimeter, less than or equal to100 microns, less than or equal to 10 microns, less than or equal to 1micron, or less than or equal to 100 nm. In some embodiments, some orall of the elongated protrusions have a cross-sectional dimension,measured parallel to the plane on which the nanostructures are arrangedand perpendicular to the length of the elongated feature, of greaterthan or equal to 10 nm. For example, referring to FIG. 2 , in someembodiments, protrusions 210B have cross-sectional dimensions indicatedby dashed lines 214. Cross-sectional dimensions 214 are measuredparallel to the top plane of substrate 102 (on which the nanostructuresare arranged) and perpendicular to the length of elongated features210B. In certain embodiments, cross-sectional dimensions 214 can be lessthan or equal to 1 millimeter (or within any of the other ranges recitedabove).

A number of electronic performance parameters of advantageous value canbe realized with devices of the invention and, as noted above, one ormore of them can experience minimal or imperceptible change in valuewhen the device is flexed or re-oriented (radius of curvature change) asdescribed above. Some of these parameters include power density, energydensity, capacitance, and the like. The invention can, in variousembodiments, achieve performance characteristics as follows:

Areal capacitance (capacitance per unit area) of at least 1.0 F/cm² at 5mA/cm², or at least 2.0 F/cm² at 5 mA/cm², or at least 3.0 F/cm² at 5mA/cm²;

Areal energy and power density of at least 0.5 mWhr/cm² and 0.5 W/cm²,respectively, or at least 0.75 mWhr/cm² and 1.0 W/cm², respectively, orat least 1.0 mWhr/cm² and 1.5 W/cm², respectively;

Areal capacitance retention with at least 0.5 F/cm², 1.0 F/cm², or 1.8F/cm² even at a current density as high as 200 mA/cm²; and/or

Areal cell capacitance retention of at least 25%, 50%, or 75% from 0.72F/cm⁻² to 0.48 F cm⁻² when increasing the current density from 5 mA/cm²to 200 mA/cm².

The devices of the invention also can achieve high current density andhigh charge discharge rates, and also high energy density, in devices asdescribed herein. The association of pseudocapacitive material withnanostructures according to embodiments described herein can alsoprovide benefits related to charging of devices comprising electrodescontaining the nanostructures, such as enhanced charging at low inputpower.

Devices that can be made using techniques of the invention includeessentially any electronic device where electrodes with good electronictransport and/or ionic conductivity interface are valued, such asbatteries, logic devices, memory devices, capacitors, includingsupercapacitors and the like.

In certain embodiments, the articles comprising theelectronically-conductive nanostructures and pseudocapacitive materialsdescribed herein further comprise a current collector in electroniccommunication with the electronically-conductive nanostructures. Thecurrent collector can, in some embodiments, comprise a catalystmaterial.

Certain embodiments are directed to capacitors. The capacitor cancomprise, in some embodiments, any of the articles described herein(e.g., an article comprising electronically-conductive nanostructuresand pseudocapacitive materials) arranged as or in an electrode (e.g., apositive electrode) in an asymmetric or a symmetric capacitor cell.

The following is a description of structures, definitions, andparameters that can be used in connection with the invention:

As used herein, the term “nanostructure” refers to an object having atleast one cross-sectional dimension of less than 1 micron. In someembodiments, the nanostructure has at least one cross-sectionaldimension of less than 500 nm, less than 250 nm, less than 100 nm, lessthan 75 nm, less than 50 nm, less than 25 nm, less than 10 nm, or, insome cases, less than 1 nm. Nanostructures described herein may have, insome cases, a maximum cross-sectional dimension of less than 1 micron,less than 500 nm, less than 250 nm, less than 100 nm, less than 75 nm,less than 50 nm, or less than 25 nm.

As used herein, the term “elongated nanostructure” refers to a structurehaving a maximum cross-sectional diameter of less than or equal to 1micron and a length resulting in an aspect ratio greater than or equalto 10. In some embodiments, the elongated nanostructure can have anaspect ratio greater than or equal to 100, greater than or equal to1000, greater than or equal to 10,000, or greater. Those skilled in theart would understand that the aspect ratio of a given structure ismeasured along the longitudinal axis of the elongated nanostructure, andis expressed as the ratio of the length of the longitudinal axis of thenanostructure to the maximum cross-sectional diameter of thenanostructure. The “longitudinal axis” of an article corresponds to theimaginary line that connects the geometric centers of the cross-sectionsof the article as a pathway is traced, along the longest length of thearticle, from one end to another.

In some cases, the elongated nanostructure may have a maximumcross-sectional diameter of less than 1 micron, less than 100nanometers, less than 50 nanometers, less than 25 nanometers, less than10 nanometers, or, in some cases, less than 1 nanometer. A “maximumcross-sectional diameter” of an elongated nanostructure, as used herein,refers to the largest dimension between two points on opposed outerboundaries of the elongated nanostructure, as measured perpendicular tothe length of the elongated nanostructure (e.g., the length of a carbonnanotube). The “average of the maximum cross-sectional diameters” of aplurality of structures refers to the number average.

The elongated nanostructure can have a cylindrical or pseudo-cylindricalshape, in some embodiments. In some embodiments, the elongatednanostructure can be a nanotube, such as a carbon nanotube. Otherexamples of elongated nanostructures include, but are not limited to,nanofibers and nanowires.

Elongated nanostructures can be single molecules (e.g., in the case ofsome nanotubes) or can include multiple molecules bound to each other(e.g., in the case of some nanofibers).

As used herein, the term “nanotube” refers to a substantiallycylindrical elongated nanostructure comprising a fused network ofprimarily six-membered rings (e.g., six-membered aromatic rings).Nanotubes may include, in some embodiments, a fused network of at least10, at least 100, at least 1000, at least 10⁵, at least 10⁶, at least10⁷, or at least 10⁸ rings (e.g., six-membered rings such assix-membered aromatic rings), or more. In some cases, nanotubes mayresemble a sheet of graphite formed into a seamless cylindricalstructure. It should be understood that the nanotube may also compriserings or lattice structures other than six-membered rings. According tocertain embodiments, at least one end of the nanotube may be capped,i.e., with a curved or nonplanar aromatic group.

Elongated nanostructures may be formed of a variety of materials, insome embodiments. In certain embodiments, the elongated nanostructurescomprise carbon (e.g., carbon-based nanostructures, described in moredetail below). Other non-limiting examples of materials from whichelongated nanostructures may be formed include silicon,indium-gallium-arsenide materials, boron nitride, silicon nitride (e.g.,Si₃N₄), silicon carbide, dichalcogenides (WS₂), oxides (e.g., titaniumdioxide, molybdenum trioxide), and boron-carbon-nitrogen compounds(e.g., BC₂N₂, BC₄N). In some embodiments, the elongated nanostructuremay be formed of one or more inorganic materials. Non-limiting examplesinclude semiconductor nanowires such as silicon (Si) nanowires,indium-gallium-arsenide (InGaAs) nanowires, and nanotubes comprisingboron nitride (BN), silicon nitride (Si₃N₄), silicon carbide (SiC),dichalcogenides such as (WS₂), oxides such as titanium dioxide (TiO₂)and molybdenum trioxide (MoO₃), and boron-carbon-nitrogen compositionssuch as BC₂N₂ and BC₄N. In certain embodiments, in instances where theelongated nanostructures are made of semiconducting or electronicallyinsulating materials, they can be coated with one or more electronicallyconductive materials to impart electronic conductivity.

As used herein, the term “carbon-based nanostructure” refers to articleshaving a fused network of aromatic rings, at least one cross-sectionaldimension of less than 1 micron, and comprising at least 30% carbon bymass. In some embodiments, the carbon-based nanostructures may compriseat least 40%, at least 50%, at least 60%, at least 70%, at least 80%, atleast 90%, or at least 95% of carbon by mass, or more. The term “fusednetwork” would not include, for example, a biphenyl group, wherein twophenyl rings are joined by a single bond and are not fused. Examples ofcarbon-based nanostructures include carbon nanotubes (e.g.,single-walled carbon nanotubes, double-walled carbon nanotubes,multi-walled carbon nanotubes, etc.), carbon nanowires, carbonnanofibers, carbon nanoshells, graphene, fullerenes, and the like. Insome embodiments, the carbon-based nanostructures comprise hollow carbonnanoshells and/or nanohorns.

In some embodiments, a carbon-based nanostructure may have at least onecross-sectional dimension of less than 500 nm, less than 250 nm, lessthan 100 nm, less than 75 nm, less than 50 nm, less than 25 nm, lessthan 10 nm, or, in some cases, less than 1 nm. Carbon-basednanostructures described herein may have, in some cases, a maximumcross-sectional dimension of less than 1 micron, less than 500 nm, lessthan 250 nm, less than 100 nm, less than 75 nm, less than 50 nm, lessthan 25 nm, less than 10 nm, or, in some cases, less than 1 nm.

According to certain embodiments, the carbon-based nanostructures areelongated carbon-based nanostructures. As used herein, the term“elongated carbon-based nanostructure” refers to a carbon-basednanostructure structure having a maximum cross-sectional diameter ofless than or equal to 1 micron and a length resulting in an aspect ratiogreater than or equal to 10. In some embodiments, the elongatednanostructure can have an aspect ratio greater than or equal to 100,greater than or equal to 1000, greater than or equal to 10,000, orgreater. As noted above, those skilled in the art would understand thatthe aspect ratio of a given structure is measured along the longitudinalaxis of the elongated nanostructure, and is expressed as the ratio ofthe length of the longitudinal axis of the nanostructure to the maximumcross-sectional diameter of the nanostructure.

In some cases, the elongated carbon-based nanostructure may have amaximum cross-sectional diameter of less than 1 micron, less than 100nanometers, less than 50 nanometers, less than 25 nanometers, less than10 nanometers, or, in some cases, less than 1 nanometer. As noted above,the “maximum cross-sectional diameter” of an elongated nanostructure, asused herein, refers to the largest dimension between two points onopposed outer boundaries of the elongated nanostructure, as measuredperpendicular to the length of the elongated nanostructure (e.g., thelength of a carbon nanotube). As noted above, the “average of themaximum cross-sectional diameters” of a plurality of structures refersto the number average.

The elongated carbon-based nanostructure can have a cylindrical orpseudo-cylindrical shape, in some embodiments. In some embodiments, theelongated carbon-based nanostructure can be a carbon nanotube. Otherexamples of elongated carbon-based nanostructures include, but are notlimited to, carbon nanofibers and carbon nanowires.

Elongated carbon-based nanostructures can be single molecules or caninclude multiple molecules bound to each other.

In some embodiments, the carbon-based nanostructures described hereinmay comprise carbon nanotubes. As used herein, the term “carbonnanotube” is given its ordinary meaning in the art and refers to asubstantially cylindrical molecule or nanostructure comprising a fusednetwork of primarily six-membered rings (e.g., six-membered aromaticrings) comprising primarily carbon atoms. In some cases, carbonnanotubes may resemble a sheet of graphite formed into a seamlesscylindrical structure. In some cases, carbon nanotubes may include awall that comprises fine-grained sp² sheets. In certain embodiments,carbon nanotubes may have turbostratic walls. It should be understoodthat the carbon nanotube may also comprise rings or lattice structuresother than six-membered rings. Typically, at least one end of the carbonnanotube may be capped, i.e., with a curved or nonplanar aromaticstructure. Carbon nanotubes may have a diameter of the order ofnanometers and a length on the order of millimeters, or, on the order oftenths of microns, resulting in an aspect ratio greater than 100, 1000,10,000, 100,000, 10⁶, 10⁷, 10⁸, 10⁹, or greater. Examples of carbonnanotubes include single-walled carbon nanotubes (SWNTs), double-walledcarbon nanotubes (DWNTs), multi-walled carbon nanotubes (MWNTs) (e.g.,concentric carbon nanotubes), inorganic derivatives thereof, organicderivatives thereof, and the like. In some embodiments, the carbonnanotube is a single-walled carbon nanotube. In some cases, the carbonnanotube is a multi-walled carbon nanotube (e.g., a double-walled carbonnanotube). In some cases, the carbon nanotube comprises a multi-walledor single-walled carbon nanotube with an inner diameter wider than isattainable from a traditional catalyst or other active growth material.In some cases, the carbon nanotube may have a diameter less than 1micron, less than 500 nm, less than 250 nm, less than 100 nm, less than50 nm, less than 25 nm, less than 10 nm, or, in some cases, less than 1nm.

As used herein, an “arrangement” of elongated nanostructures correspondsto a plurality of elongated nanostructures arranged in relation to eachother as described herein. In some embodiments, the arrangement ofelongated nanostructures comprises at least 5, at least 10, at least 50,at least 100, at least 500, at least 1000, or at least 10,000 elongatednanostructures. In some such embodiments, the arrangement of elongatednanostructures may comprise at least 10⁶, at least 10⁷, at least 10⁸, atleast 10⁹, at least 10¹⁰, at least 10¹¹, at least 10¹², or at least 10¹³elongated nanostructures. Those of ordinary skill in the art arefamiliar with suitable methods for forming arrangements of elongatednanostructures with benefit of the disclosure provided here. Forexample, in some embodiments, the arrangement of elongatednanostructures can be catalytically grown (e.g., using a growth catalystdeposited via chemical vapor deposition process).

In some embodiments in which the nanostructures are grown on asubstrate, the set of substantially aligned nanostructures may beoriented such that the longitudinal axes of the nanostructures aresubstantially non-parallel to the surface of the growth substrate. Insome cases, the longitudinal axes of the nanostructures are oriented ina substantially perpendicular direction with respect to the surface ofthe growth substrate. As described more fully below, an advantageousfeature of some embodiments of the invention may be that the alignmentof nanostructures in the nanostructure forest may be substantiallymaintained, even upon subsequent processing (e.g., application of aforce to the arrangement, transfer of the arrangement to other surfaces,and/or combining the arrangements with secondary materials such aspolymers, metals, ceramics, piezoelectric materials, piezomagneticmaterials, carbon, and/or fluids, among other materials).

Systems and methods for growing elongated nanostructures (includingforests of elongated nanostructures) are described, for example, inInternational Patent Application Serial No. PCT/US2007/011914, filed May18, 2007, entitled “Continuous Process for the Production ofNanostructures Including Nanotubes,” published as WO 2007/136755 on Nov.29, 2007; U.S. patent application Ser. No. 12/227,516, filed Nov. 19,2008, entitled “Continuous Process for the Production of NanostructuresIncluding Nanotubes,” published as US 2009/0311166 on Dec. 17, 2009;International Patent Application Serial No. PCT/US07/11913, filed May18, 2007, entitled “Nanostructure-reinforced Composite Articles andMethods,” published as WO 2008/054541 on May 8, 2008; InternationalPatent Application Serial No. PCT/US2008/009996, filed Aug. 22, 2008,entitled “Nanostructure-reinforced Composite Articles and Methods,”published as WO 2009/029218 on Mar. 5, 2009; U.S. patent applicationSer. No. 11/895,621, filed Aug. 24, 2007, entitled“Nanostructure-Reinforced Composite Articles and Methods,” published asUS 2008/0075954 on Mar. 27, 2008; and U.S. Patent Publication No.2010/0196695, published on Aug. 5, 2010, and filed as application Ser.No. 12/618,203 on Nov. 13, 2009; each of which is incorporated herein byreference in its entirety for all purposes. These “forests” can bere-arranged, as described herein, such that a majority of thenanostructures include a longest dimension oriented substantiallyparallel to the surface, and other orientations described herein.

For a given elongated nanostructure in an arrangement of elongatednanostructures, the “nearest neighbor” corresponds to the elongatednanostructure having a longitudinal axis that is closest to thelongitudinal axis of the given elongated nanostructure at any pointalong the longitudinal axis of the given elongated nanostructure.

In certain embodiments, the arrangement of elongated nanostructures hasa number average of nearest neighbor distances that is less than 2.5%,less than 1%, less than 0.5%, less than 0.25%, less than 0.1%, or lessthan 0.05% of the average length of the elongated nanostructures withinthe arrangement. For example, as illustrated in FIG. 1B, an arrangementof elongated nanostructures 110 may have a nearest neighbor distancebetween two elongated nanostructures 135 (e.g., between elongatednanostructure 110 and elongated nanostructure 110 a) and an averagelength 130. In some embodiments, the number average of nearest neighbordistances within the arrangement of elongated nanostructures is lessthan 250 nanometers, less than 200 nanometers, less than 150 nanometers,less than 100 nanometers, less than 50 nanometers, less than 25nanometers, less than 10 nanometers, or less than 5 nanometers. Incertain embodiments, the number average of nearest neighbor distanceswithin the arrangement of elongated nanostructures is greater than orequal to 2 nanometers, greater than or equal to 5 nanometers, greaterthan or equal to 10 nanometers, greater than or equal to 25 nanometers,greater than or equal to 50 nanometers, greater than or equal to 100nanometers, greater than or equal to 150 nanometers, or greater than orequal to 200 nanometers. Combinations of the above-referenced ranges arealso possible (less than 250 nanometers and greater than or equal to 2nanometers). Other ranges are also possible. The number average ofnearest neighbor distances within the arrangement of elongatednanostructures may be calculated by determining the nearest neighbordistance for each nanostructure, then number averaging the nearestneighbor distances. Nearest neighbor distances of the elongatednanostructures can be determined by 2- and 3-dimensional scanning andtransmission electron tomography.

In some embodiments, the nearest neighbor distance within thearrangement is roughly equal for each nanostructure. For example, asillustrated in FIG. 1B, nearest neighbor distance 135 is roughly equalbetween all nearest neighbor elongated nanostructures in thearrangement. In other embodiments, the nearest neighbor distances foreach elongated nanostructure may vary.

In some embodiments, the arrangement of elongated nanostructures extendsa distance, in each of two orthogonal directions each perpendicular tothe longitudinal axes of the nanostructures, that is at least 10 timesgreater than the number average of nearest neighbor distances within thearrangement.

In some cases, the arrangement of elongated nanostructures extends, intwo orthogonal directions each perpendicular to the long axes, adistance at least 100 times greater, at least 1000 times greater, atleast 10,000 times greater or longer than the number average of thenearest neighbor distances within the arrangement. In certainembodiments, the arrangement of elongated nanostructures extends, in atleast one of two orthogonal directions each perpendicular to the longaxes, a distance at least 10⁶ times, at least 10⁷ times 10⁸ times, atleast 10⁹ times, or at least 10¹⁰ times greater or longer than thenumber average of nearest neighbor distances within the arrangement.

In some cases, an arrangement of elongated nanostructures may beprovided such that the arrangement extends, in at least one dimension(e.g., in one dimension, in two orthogonal dimensions, etc.)substantially perpendicular to the long axes, a distance at least 1.5times greater, at least 2 times greater, at least 5 times greater, atleast 10 times greater, at least 25 times greater, at least 100 timesgreater, or more than a dimension substantially parallel to thelongitudinal axes of the elongated nanostructures. As a specificexample, the arrangement of elongated nanostructures may constitute athin-film such that the longitudinal axes of the nanostructures aresubstantially perpendicular to the largest surface of the film.

An arrangement of elongated nanostructures may be provided, in someinstances, such that the arrangement extends, in at least one dimensionsubstantially parallel to the long axes (e.g., length dimension 130 inFIG. 1B), a distance at least 1.5 times greater, at least 2 timesgreater, at least 5 times greater, at least 10 times greater, at least25 times greater, at least 100 times greater, or more than a dimensionsubstantially perpendicular to the long axes of the elongatednanostructures (e.g., dimension 180 in FIG. 1B). In an alternativeembodiment, the arrangement of elongated nanostructures may be providedsuch that the arrangement extends, in at least one dimensionsubstantially perpendicular to the long axes 170 (e.g., dimension 180 inFIG. 1B), a distance at least 1.5 times greater, at least 2 timesgreater, at least 5 times greater, at least 10 times greater, at least25 times greater, at least 100 times greater, or more than a dimensionsubstantially parallel to the long axes 170 of the elongatednanostructures.

According to certain embodiments, the elongated nanostructures withinthe arrangement are substantially aligned. Alignment of the elongatednanostructures as described herein can be determined by 3-dimensionalelectron tomography.

In some embodiments, at least 10%, at least 20%, at least 30%, at least40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least90% of the elongated nanostructures are parallel to within 30 degrees,within 20 degrees, within 10 degrees, within 5 degrees, or within 2degrees of a common vector. Those skilled in the art would understandthat elongated nanostructures may have some inherent deviation alongtheir length such as waviness. Accordingly, for the purposes ofdetermining the alignment of elongated nanostructures with respect to acommon vector, one would draw a line from one end of the elongatednanostructure to the other end of the elongated nanostructure.

In some embodiments, the elongated nanostructures within the arrangementmay be closely spaced. For example, the number average of the nearestneighbor distances of the elongated nanostructures within thearrangement may be less than 250 nm, less than 200 nm, less than 100 nm,less than 80 nm, less than 60 nm, less than 40 nm, less than 30 nm, lessthan 20 nm, less than 10 nm, less than 5 nm, or less. In certainembodiments, the number average of the nearest neighbor distances of theelongated nanostructures within the arrangement may be at least 1 nm, atleast 5 nm, at least 10 nm, at least 20 nm, at least 30 nm, at least 40nm, at least 60 nm, at least 80 nm, at least 100 nm, or at least 200 nm.Combinations of the above-referenced ranges are also possible (e.g., atleast 1 nm and less than 250 nm). Other ranges are also possible.

In some cases, the nanostructure materials or the nanocomposites maycomprise a high volume fraction of nanostructures. For example, thevolume fraction of the nanostructures within the materials may be atleast 10%, at least 20%, at least 40%, at least 60%, at least 70%, atleast 75%, at least 78%, or higher.

In some cases, the nanostructures are dispersed substantially uniformlywithin a hardened support material. For example, the nanostructures maybe dispersed substantially uniformly within at least 10% of the hardenedsupport material, or, in some cases, at least 20%, 30%, 40%, 50%, 60%,70%, 80%, 90%, 95%, or 100% of the hardened support material. As usedherein, “dispersed uniformly within at least X % of the hardened supportmaterial” refers to the substantially uniform arrangement ofnanostructures within at least X % of the volume of the hardened supportmaterial. The ability to arrange nanostructures essentially uniformlythroughout structures comprising plurality of fibers allows for theenhanced mechanical strength of the overall structure.

In certain embodiments, the elongated nanostructures described hereinhave relatively low geometric tortuosities. For example, in certainembodiments, the arrangement of elongated nanostructures comprises someelongated nanostructures (e.g., at least 10, at least 25, at least 50,at least 100, or at least 1000 elongated nanostructures) with geometrictortuosities of less than 3, less than 2.5, less than 2, less than 1.5,less than 1.2, or less than 1.1 (and, in certain embodiments, down tosubstantially 1). The geometric tortuosity of a particular elongatednanostructure is calculated as the effective path length divided by theprojected path length. One of ordinary skill in the art would be capableof determining the geometric tortuosity of a given elongatednanostructure by examining an image (e.g., a magnified image such as ascanning electron micrograph, a microscope enhanced photograph, or anunmagnified photograph), determining the effective path length bytracing a pathway from one end of the elongated nanostructure to theother end of the elongated nanostructure along the longitudinal axis ofthe elongated nanostructure, and determining the projected path lengthby measuring the straight-line distance between the ends of theelongated nanostructure.

According to certain embodiments, the arrangement of elongatednanostructures has an average tortuosity of less than 3, less than 2.5,less than 2, less than 1.5, less than 1.2, or less than 1.1 (and, incertain embodiments, down to substantially 1). The average tortuosity ofan arrangement of elongated nanostructures is calculated as the numberaverage of the tortuosities of the individual elongated nanostructures.

According to certain embodiments, at least 50%, at least 75%, at least90%, at least 95%, or at least 99% of the elongated nanostructures inthe arrangement of elongated nanostructures have a tortuosity of lessthan 3, less than 2.5, less than 2, less than 1.5, less than 1.2, orless than 1.1 (and, in certain embodiments, down to substantially 1).

According to certain embodiments, the arrangement of elongatednanostructures comprises elongated nanostructures having lengths of atleast 5 microns, at least 10 microns, at least 100 microns, at least 1mm, at least 5 mm, at least 10 mm, or at least 100 mm (and/or, incertain embodiments, up to 200 mm, up to 500 mm, up to 1 m, or longer).According to some embodiments, at least 50%, at least 75%, at least 90%,at least 95%, or at least 99% of the elongated nanostructures in thearrangement of elongated nanostructures have lengths of at least 5microns, at least 10 microns, at least 100 microns, at least 1 mm, atleast 5 mm, at least 10 mm, or at least 100 mm (and/or, in certainembodiments, up to 200 mm, up to 500 mm, up to 1 m, or longer).

In some embodiments, an article or device described herein may exhibit arelatively large elastic modulus. In some cases, an arrangement mayexhibit a relatively large elastic modulus in a particular direction.For example, in some embodiments, the composite article exhibits arelatively large elastic modulus measured in a direction substantiallyparallel to the longitudinal axes of the elongated nanostructures. Acomposite material comprising the elongated nanostructures may have,according to certain but not necessarily all embodiments, improvedmechanical properties (e.g., increased strength, increased strain tofailure, increased toughness, increased elastic modulus) as compared tothe support material alone in a direction substantially parallel to thelongitudinal axes of the elongated nanostructures. In certainembodiments comprising an arrangement of elongated nanostructureslocated at the interface between a first substrate and a secondsubstrate, the arrangement may exhibit increased interlaminarreinforcement across the first and second substrates.

For example, in one set of embodiments, an arrangement (e.g., anarrangement comprising an arrangement of elongated nanostructures) mayhave an elastic modulus of at least 100 MPa, at least 500 MPa, at least1 GPa, at least 5 GPa, at least 7.5 GPa, at least 10 GPa, at least 100GPa, or at least 500 GPa, or higher, measured in a directionsubstantially parallel to the longitudinal axes of the elongatednanostructures. In some cases, an elastic modulus (in one or moredirections) of an arrangement may be at least 2%, at least 5%, 10%, atleast 25%, at least 50%, at least 100%, at least 200%, at least 500%, orat least 1000% larger than the elastic modulus that would be exhibitedby the support material absent the arrangement of elongatednanostructures, but under otherwise essentially identical conditions. Inthis context, essentially identical conditions means that the supportmaterial, temperature, dimensions, and other parameters of the structureand testing procedure would be substantially the same as the compositematerial, but the arrangement of elongated nanostructures would not bepresent. The elastic modulus, as described herein, can be determinedusing a nanoindenter (e.g., a Nanotest 600 nanomechanical testing system(Micro Materials, UK)) with a Berkovich-type indenter inside thenanoindenter's thermally insulated environmental chamber (25° C.±0.5°C., relative humidity 45%±2%) at a loading and unloading rate of 100mN/s.

In some embodiments, an arrangement of nanostructures, optionally coatedand provided on a substrate surface described herein may exhibit arelatively large electrical conductivity. In some cases, an article mayexhibit a relatively large electrical conductivity in a particulardirection. For example, in some embodiments, the article exhibits arelatively large electrical conductivity measured in a directionsubstantially parallel to the longitudinal axes of the elongatednanostructures. A material comprising the arrangement of elongatednanostructures may have, according to certain but not necessarily allembodiments, improved electrical properties (e.g., increased electricalconductivity) as compared to the support material alone in a directionsubstantially parallel to the longitudinal axes of the elongatednanostructures. In some embodiments, an arrangement (e.g., anarrangement comprising an arrangement of elongated nanostructures) mayhave an electrical conductivity of at least 10⁻⁴ S/m, 10⁻³ S/m, 10⁻²S/m, 0.1 S/m, 1 S/m, 10 S/m, 100 S/m, 10³ S/m, 10⁴ S/m, or greater. Insome cases, the electrical conductivity of a composite material may beat least 5 times, at least 10 times, at least 50 times, at least 100times, at least 1000 times, at least 10,000 times, at least 100,000times, at least 1,000,000 times, at least 10,000,000 times, or at least100,000,000 times larger than the electrical conductivity that would beexhibited by the support material absent the elongated nanostructures,but under otherwise essentially identical conditions. In this context,essentially identical conditions means that the support material,temperature, dimensions, and other parameters of the structure andtesting procedure would be substantially the same as the compositematerial, but the elongated nanostructures would not be present.Electrical conductivity, as described herein, may be determined usingdirect current impedance measurements.

As noted above, certain embodiments are related to methods of growingcarbon-based nanostructures. According to some embodiments, the methodof growing carbon-based nanostructures comprises providing an activegrowth material or an active growth material precursor and exposing aprecursor of the carbon-based nanostructures to the active growthmaterial or active growth material precursor. It should be understoodthat, where active growth materials and their associated properties aredescribed below and elsewhere herein, either or both of the activegrowth material itself and the active growth material precursor may havethe properties described as being associated with the active growthmaterial. In some embodiments, the active growth material and/or theactive growth material precursor has these properties upon being exposedto the carbon-based nanostructure precursor. In certain embodiments, theactive growth material and/or the active growth material precursor hasthese properties at the beginning of a heating step used to form thecarbon-based nanostructures. In certain embodiments, the active growthmaterial and/or the active growth material precursor has theseproperties at least one point in time during which the material is in achamber or other vessel within which the carbon-based nanostructures aregrown.

It should be understood that the growth of carbon-based nanostructurescan include the initial nucleation/formation of the carbon-basednanostructure and/or making an existing carbon-based nanostructurelarger in size. In certain embodiments, the growth of the carbon-basednanostructures comprises nucleating or otherwise forming thecarbon-based nanostructures from material that is not a carbon-basednanostructure. In some embodiments, two or more carbon-basednanostructures may nucleate or otherwise form from a material that isnot a carbon-based nanostructure. The two or more carbon-basednanostructures may be the same type of carbon-based nanostructure, ormay be different types of carbon-based nanostructures. In someembodiments, the growth of the carbon-based nanostructures comprisesmaking an existing carbon-based nanostructure larger in size. The growthprocess can also include both of these steps, in some cases. In certainembodiments, multiple growth steps can be performed, for example, usinga single active growth material to grow carbon-based nanostructuresmultiple times.

The precursor of the carbon-based nanostructures can be exposed to theactive growth material in a number of ways. Generally, exposing theactive growth material to the precursor comprises combining theprecursor and the active growth material with each other such that theyare in contact. According to certain embodiments, exposing the precursorof the carbon-based nanostructures to the active growth materialcomprises adding the precursor of the carbon-based nanostructures to theactive growth material. In certain embodiments, exposing the precursorof the carbon-based nanostructures to the active growth materialcomprises adding the active growth material to the precursor of thecarbon-based nanostructures. In still other embodiments, the precursorof the carbon-based nanostructures and the active growth material can bemixed simultaneously. Other methods of exposure are also possible.Exposing the precursor of the carbon-based nanostructures to an activegrowth material can occur, according to some embodiments, in a chamberor other volume. The volume in which the precursor of the carbon-basednanostructures is exposed to the active growth material may be fullyenclosed, partially enclosed, or completely unenclosed.

According to certain embodiments, carbon from the precursor of thecarbon-based nanostructures may be dissociated from the precursor. Thedissociation of the carbon from the precursor can, according to certainembodiments, involve the breaking of at least one covalent bond. Inother cases, the dissociation of the carbon from the precursor does notinvolve breaking a covalent bond. The carbon dissociated from theprecursor may, according to certain embodiments, chemically react togrow the carbon-based nanostructures via the formation of new covalentbonds (e.g., new carbon-carbon covalent bonds). In the growth of carbonnanotubes, for example, the nanostructure precursor material maycomprise carbon, such that carbon dissociates from the precursormolecule and may be incorporated into the growing carbon nanotube viathe formation of new carbon-carbon covalent bonds.

As described in more detail below, a variety of materials can be used asthe precursor of the carbon-based nanostructures and as the activegrowth material (or a precursor of the active growth material).According to certain embodiments, carbon-based nanostructures (e.g.,carbon nanotubes) may be synthesized using CO₂ and acetylene asprecursors of the carbon-based nanostructures. Other examples ofnanostructure precursor materials, active growth materials, precursorsof active growth materials, and the types of carbon-based nanostructuresthat may be grown using these materials are described in more detailbelow.

In some embodiments, the method of growing carbon-based nanostructurescomprises exposing the active growth material and the precursor of thecarbon-based nanostructures to a set of conditions that causes growth ofcarbon-based nanostructures on the active growth material. Growth of thecarbon-based nanostructures may comprise, for example, heating theprecursor of the carbon-based nanostructures, the active growthmaterial, or both. Other examples of suitable conditions under which thecarbon-based nanostructures may be grown are described in more detailbelow. In some embodiments, growing carbon-based nanostructurescomprises performing chemical vapor deposition (CVD) of nanostructureson the active growth material. In some embodiments, the chemical vapordeposition process may comprise a plasma chemical vapor depositionprocess. Chemical vapor deposition is a process known to those ofordinary skill in the art, and is explained, for example, in DresselhausM S, Dresselhaus G., and Avouris, P. eds. “Carbon Nanotubes: Synthesis,Structure, Properties, and Applications” (2001) Springer, which isincorporated herein by reference in its entirety. Examples of suitablenanostructure fabrication techniques are discussed in more detail inInternational Patent Application Serial No. PCT/US2007/011914, filed May18, 2007, entitled “Continuous Process for the Production ofNanostructures Including Nanotubes,” published as WO 2007/136755 on Nov.29, 2007, which is incorporated herein by reference in its entirety.

In some cases, the nanostructures may be removed from a substrate afterthe nanostructures are formed. For example, the act of removing maycomprise transferring the nanostructures directly from the surface ofthe substrate to a surface of a receiving substrate. The receivingsubstrate may be, for example, a polymer material or a carbon fibermaterial. In some cases, the receiving substrate comprises a polymermaterial, metal, or a fiber comprising Al₂O₃, SiO₂, carbon, or a polymermaterial. In some cases, the receiving substrate comprises a fibercomprising Al₂O₃, SiO₂, carbon, or a polymer material. In someembodiments, the receiving substrate is a fiber weave.

Removal of the nanostructures may comprise application of a mechanicaltool, mechanical or ultrasonic vibration, a chemical reagent, heat, orother sources of external energy, to the nanostructures and/or thesurface of the growth substrate. In some cases, the nanostructures maybe removed by application of compressed gas, for example. In some cases,the nanostructures may be removed (e.g., detached) and collected inbulk, without attaching the nanostructures to a receiving substrate, andthe nanostructures may remain in their original or “as-grown”orientation and conformation (e.g., in an aligned “forest”) followingremoval from the growth substrate. Systems and methods for removingnanostructures from a substrate, or for transferring nanostructures froma first substrate to a second substrate, are described in InternationalPatent Application Serial No. PCT/US2007/011914, filed May 18, 2007,entitled “Continuous Process for the Production of NanostructuresIncluding Nanotubes,” which is incorporated herein by reference in itsentirety.

In some embodiments, the active growth material may be removed from thegrowth substrate and/or the nanostructures after the nanostructures aregrown. Active growth material removal may be performed mechanically, forexample, via treatment with a mechanical tool to scrape or grind theactive growth material from a surface (e.g., of a substrate). In somecases, the first active growth material may be removed by treatment witha chemical species (e.g., chemical etching) or thermally (e.g., heatingto a temperature which evaporates the active growth material). Forexample, in some embodiments, the active growth material may be removedvia an acid etch (e.g., HCl, HF, etc.), which may, for example,selectively dissolve the active growth material. For example, HF can beused to selectively dissolve oxides. In some embodiments, the firstactive growth material may be removed by a combination of treatment witha chemical species and treatment with heat (e.g., the first activegrowth material may be heated in the presence of H₂). When heating isemployed to remove the first active growth material, it may be appliedby exposing the active growth material to a heated environment and/or byusing an electron beam to heat the active growth material.

While growth of nanostructures using a growth substrate has beenprimarily described above in detail, the embodiments described hereinare not so limited, and carbon-based nanostructures may be grown, insome embodiments, on an active growth material in the absence of agrowth substrate. For example, active growth material can be placedunder a set of conditions selected to facilitate nanostructure growth inthe absence of a substrate in contact with the active growth material.Nanostructures may grow from active growth material as the active growthmaterial is exposed to the growth conditions. In some embodiments, theactive growth material, or a precursor thereof, may be suspended in afluid. For example, an active growth material, or a precursor thereof,may be suspended in a gas (e.g., aerosolized) and subsequently exposedto a carbon-containing precursor material, from which carbon nanotubesmay be grown. In some cases, the active growth material, or a precursorthereof, may be suspended in a liquid (e.g., an alcohol that serves as ananostructure precursor material) during the formation of thenanostructures. In some embodiments, unsupported active growthmaterials, or precursors thereof, are in contact with a gas or vacuum atevery point comprising their surfaces. Active growth materials having avariety of shapes are contemplated, including hemispherical shapes,spherical shapes, polygonal shapes, and the like.

As used herein, “nanostructure,” refers to an article with at least onecross-sectional dimension of less than 1 micron. In some cases, thenanostructure may have at least one cross-sectional dimension of lessthan 500 nm, less than 250 nm, less than 100 nm, less than 10 nm, lessthan 5 nm, less than 3 nm, less than 2 nm, less than 1 nm, between 0.3and 10 nm, between 10 nm and 100 nm, or between 100 nm and 1 micron.

In some embodiments, the substrate may comprise carbon (e.g., amorphouscarbon, carbon aerogel, carbon fiber, graphite, glassy carbon,carbon-carbon composite, graphene, aggregated diamond nanorods,nanodiamond, diamond, and the like).

In accordance with certain embodiments, the substrate (e.g., on whichthe active growth material, or a precursor thereof, and/or thecarbon-based nanostructures are supported) comprises a fiber. Forexample, in some embodiments, the active growth material, active growthmaterial precursor, and/or carbon-based nanostructures are supported ona carbon fiber. In certain embodiments, the active growth material,active growth material precursor, and/or carbon-based nanostructures aresupported on a glass fiber. In accordance with some embodiments, theactive growth material, active growth material precursor, and/orcarbon-based nanostructures are supported on fibers comprising one ormore of the following materials: carbon; carbon glass; glass; alumina;basalt; metals (e.g., steel, aluminum, titanium); aramid (e.g., Kevlar®,meta-aramids such as Nomex®, p-aramids); liquid crystalline polyester;poly(p-phenylene-2,6-benzobisoxazole) (PBO); polyethylene (e.g.,Spectra®);poly{2,6-diimidazo[4,5-b:4′,5′-e]pyridinylene-1,4-(2,5-dihydroxy)phenylene};and combinations of these. In some embodiments, the active growthmaterial, active growth material precursor, and/or carbon-basednanostructures are supported on fibers comprising at least one ofpolyetherether ketone (PEEK) and polyether ketone (PEK). For example, inFIG. 1A, substrate 102 is an elongated substrate, and can correspond to,for example, a fiber such as a carbon fiber. Substrate 102 can be indirect contact with an active growth material. In some such embodiments,carbon-based nanostructures 110 can be grown from a precursor on anactive growth material.

As noted above, in some embodiments, the active growth material, activegrowth material precursor, and/or carbon based nanostructures aresupported on a carbon fiber (e.g., a sized carbon fiber or an unsizedcarbon fiber). Any suitable type of carbon fiber can be employedincluding, for example, aerospace-grade carbon fibers, auto/sport gradecarbon fibers, and/or microstructure carbon fibers. In certainembodiments, intermediate modulus (IM) or high modulus (HM) carbonfibers can be employed. In some embodiments, poly(acrylonitrile)-derivedcarbon fibers can be employed. Certain embodiments of the invention areadvantageous for use with carbon fibers that carry a large degree oftheir tensile strengths in their outer skins (e.g., fibers in which atleast 50%, at least 75%, or at least 90% of the tensile strength isimparted by the portion of the fiber located a distance away from theouter skin of the fiber of less than 0.1 times or less than 0.05 timesthe cross-sectional diameter of the fiber), such as aerospace gradeintermediate modulus carbon fibers.

According to certain embodiments, the growth substrate can be a prepreg.

In certain embodiments, the substrate can be a carbon-based substrate.In some embodiments, the carbon-based growth substrate contains carbonin an amount of at least 75 wt %, at least 90 wt %, at least 95 wt %, orat least 99 wt %. That is to say, in some embodiments, at least 75 wt %,at least 90 wt %, at least 95 wt %, or at least 99 wt % of thecarbon-based growth substrate is made of carbon.

According to certain embodiments, the thickness of the pseudocapacitivematerial can be relatively consistent over the underlyingnanostructures. For example, in certain embodiments, the thickness ofthe pseudocapacitive material, over at least 80% of the surface area ofthe nanostructure that is covered by the material, does not deviate fromthe average thickness of the material by more than 50%, more than 40%,more than 30%, more than 20%, more than 10%, or more than 5%. In someembodiments, the thickness of the material, over at least 90% (or atleast 95%, at least 98%, or at least 99%) of the surface area of thenanostructure that is covered by the material, does not deviate from theaverage thickness of the material by more than 50%, more than 40%, morethan 30%, more than 20%, more than 10%, or more than 5%.

The average thickness of the pseudocapacitive material coating can be,according to certain embodiments, relatively thin. For example,according to certain embodiments, the average thickness of the materialcan be less than 1 micron, less than 500 nm, less than 200 nm, less than100 nm, less than 50 nm, or less than 10 nm (and/or, in certainembodiments, as little as 1 nm, as little as 0.1 mm, or less).

The following example is intended to illustrate certain embodiments ofthe present invention, but does not exemplify the full scope of theinvention.

Example

This example describes the fabrication of flexible densifiedhorizontally-aligned CNT arrays (HACNTs) having conformally-coatedpoly(3-methylthiophene) (P3MT) conducting polymer to impartpseudocapacitance.

In this example, a two-step method to fabricate highly flexibledensified horizontally-aligned CNT arrays (HACNTs)/conformally-coatedpoly(3-methylthiophene) (P3MT) CP as supercapacitor electrodes with atailored nanoporous morphology was employed. Starting withvertically-aligned CNTs (A-CNTs), a one-step mechanical rollingtreatment was used to create horizontally-aligned CNTs (HACNTs). Next, adry process via modified oxidative chemical vapor deposition (o-CVD) wasemployed to conformally deposit P3MT on the HACNTs. It is noted that thealignment in the CNTs network was preserved after polymer deposition,preserving the aligned channels between HACNTs coated with CP, whichenhances the conductivity and electrochemical performances of theas-assembled cell, as expected. In this configuration, P3MT/HACNTflexible electrodes show extremely high areal capacitance of 3.1 F cm⁻²at 5 mA cm⁻² compared with those in most extant works. Thanks to thealigned ionic transport channels, the electrodes also exhibit high arealcapacitance retention with 1.8 F cm⁻² even at a current density as highas 200 mA cm⁻². Such performance retention at high current density hasnot been previously reported. The composite electrode was then used toassemble a flexible asymmetric supercapacitor cell with much higher cellcapacitance, energy and power densities compared to moststate-of-the-art flexible cells. Furthermore, due to the high mechanicalstrength of HACNTs, the electrochemical performances of the cell changedvery little upon severe bending, demonstrating real potentialapplication in flexible and wearable electronics.

Results and Discussion

The fabrication process of the flexible P3MT/HACNTs electrode isillustrated in FIG. 3A. A-CNTs are synthesized by thermal catalyticchemical vapor deposition (CVD) on silicon wafers, which was reported inprior works. (See, e.g., B. L. Wardle, et al., Advanced Materials,“Fabrication and characterization of ultrahigh-volume-fraction alignedcarbon nanotube-polymer composites,” 2008, 20, 2707-2714; and I. Y.Stein et al., “Mesoscale evolution of non-graphitizing pyrolytic carbonin aligned carbon nanotube carbon matrix nanocomposites,” Journal ofMaterials Science, 2008, 52, 13799-13811, each of which is incorporatedherein by reference in its entirety for all purposes.) These CNT arrayshave a highly aligned nanostructure with approximately 1% volumefraction (V_(f)), corresponding to ˜80 nm inter-CNT spacing and anaverage CNT diameter of 8 nm. Then a facile one-step treatment wasemployed to convert the vertical aligned CNT arrays to horizontallyaligned CNT arrays (HACNTs). In this process, a Teflon roller or razorblade was used to roll the CNT arrays along the horizontal alignmentdirection as shown in the middle of FIG. 3A. Meanwhile, CNT arrays havebeen densified by ˜12× (V_(f)=12 vol %) as determined by the thicknessafter densification. Based on some prior studies, increasing the densityof the nanostructured electrode gives rise to high volumetricelectrochemical performance. (See Y. Zhou et al., Electrochimica Acta,2013, 111, 608-613.) To increase the areal capacitance, P3MT CP wasdeposited conformally on the CNT via oCVD process. oCVD is a uniquecoating technique for the deposition of CPs not only because nearly anysubstrate can be used (unlike electrochemical deposition which requireselectrically conductive substrate) but also it can lead to nanoscalecontrollable homogeneous and conformal coatings. (See, e.g., N. Lachmanet al., “Tailoring thickness of conformal conducting polymer decoratedaligned carbon nanotube electrodes for energy storage,” AdvancedMaterials Interfaces, 2014, 1, 1400076, which is incorporated herein byreference in its entirety for all purposes.). During the oCVD process,the coated polymerized thin films can be formed via simultaneousexposure to vapor-phase monomer (3MT) and oxidant (FeCl₃) without theneed for either a solvent or the potential/current used inelectrochemical deposition. P3MT is a very promising new CP forsupercapacitor electrodes although very few works have been reported.Based on the chemical structure in FIG. 3B, with only one methyl as theside group of the thiophene ring, P3MT demonstrates higher density ofeffective conjugation (polythiophene structure) than other modifiedpolythiophene-like polymers, such as PEDOT and poly(3-hexylthiophene)(P3HT). Therefore, the P3MT CP coating is well-suited chemically toenhance the electrochemical performance of the composite electrode. Inaddition, 3-methyl thiophene also has much lower vapor pressure comparedto thiophene, which significantly facilitates the adsorption of monomermolecules on the substrate in the oCVD process. FIG. 3A also illustratesthe difference in the ion storage and transport processes between theuncoated HACNTs and the P3MT/HACNTs. In the HACNTs electrodes, the ionsare only stored on the surface of A-CNTs (electrochemical double layertype) and excess ions will travel through the ion pathways formed by theHACNTs during the charge/discharge processes. By contrast, in theP3MT/HACNTs electrodes, the ions travel into the redox material (P3MTfilms) during the charging process to store more ions based on thepseudocapacitive working mechanism, leading to increased specificcapacitance and energy density. In addition, the P3MT/HACNTs electrodealso shows excellent bendable and rollable properties as shown in thedigital images in FIGS. 3C and 3D. This flexibility can be attributed tothe HACNT strength as well as the conformal coating of CP via the uniqueoCVD process.

The nanomorphology of HACNTs and P3MT/HACNTs nanocomposite wasinvestigated by scanning electron microscopy (SEM) and Transmissionelectron microscopy (TEM) with exemplary images shown in FIGS. 4A and4B, and FIG. 4C respectively. Based on the images, one can clearly findthat the P3MT was conformally coated on the nanotubes and thehorizontally aligned CNTs are maintained after oCVD process. Thediameter of the nanofibers increased to ˜18 nm after coating and hencethe coating thickness of P3MT can be determined as ˜5 nm considering thesynthesized CNT has diameter of 8 nm. The consequence of thickernanofibers after coating corresponds to a decrease in the averagespacing between the nanofibers in the densified HACNTs from 23 nm to 5nm, which allows ion transport and provides enough space for volumechange of the CP during the doping and de-doping pseudocapacitanceprocesses. This conformal and homogeneous coating around the HACNTs ishighly desirable for the electrochemical performance of assembledsupercapacitor cell since it allows for a stress-freelarge-volume/strain change of CP during the charging/dischargingprocesses, avoiding collapse of the electrodes. Hence, this uniqueelectrode nanomorphology can support long cycling times. The compositephase and structural features were also characterized by Ramanspectroscopy. As shown in FIG. 4D, two prominent peaks can be found at˜1307 cm⁻¹ and 1602 cm⁻¹, representing the D and G bands for CNTs. The Dband expresses the vibration of the disordered carbon structure while Gband shows the sp²-bonded carbon atoms. The peak at 1430 cm⁻¹ in Ramanspectra can be attributed to the presence of P3MT.

The electrochemical properties of the P3MT/HACNTs composite electrodeand HACNTs electrode were then characterized in a three-electrodeconfiguration using 1 M tetraethylammonium tetrafluoroborate/propylenecarbonate (Et4NBF4/PC) as the electrolyte. As a comparison, CNTbuckypapers that have been utilized as flexible supercapacitor electrodesubstrates in state-of-the-art works were also synthesized based on aprocedure reported before. (See M. Endo et al. Nature, 2005, 433, 476.)As shown in the SEM image in FIG. 8 , contrary to our HACNTs, the CNTsin the buckypaper are randomly dispersed. FIG. 5A presents the cyclicvoltammetry (CV) scan curves of these three electrode types at a scanrate of 100 mV s⁻¹. The CV curves of HACNTs and buckypaper electrodesshow stable electrochemical windows (ECW) between −2 V to 1 V, while theCV curve of the composite electrode shifted and expanded to between −1.7V to 1.5 V due to the pseudocapacitive redox reactions dominated fromP3MT during the charging/discharging process. The small peaks in the CVcurve of the nanocomposite P3MT/HACNTs electrode in FIG. 5A is a furtherindicator of the redox reactions. Based on the enclosed area of the CVcurves, the areal capacitance of HACNTs is ˜2× greater than that ofbuckypaper, indicating the superior properties of the well-alignednanostructure of HACNTs. The composite electrode exhibits ˜2× and ˜8×greater in areal capacitance compared with HACNTs and buckypaperrespectively, which can be due to the high capacitance of the novel P3MTCP, as shown in FIG. 5A.

Galvanostatic charge/discharge (GCD) tests were then carried out on thethree electrodes under various current densities. FIG. 9 shows the GCDcurves at alternating current densities of 10 mA cm⁻² and −10 mA cm⁻².The areal capacitance can be calculated by the equation C=I/(dV/dt)(where 1 is the discharge current density; t is the discharge time and Vis the potential) and the areal capacitance obtained as exhibited inFIG. 5B. Consistent with the data from the CV curves, P3MT/HACNTsnanocomposite electrodes exhibits the highest areal capacitance whilethe lowest one was achieved for the buckypaper. The buckypaper alsoexhibits relatively poor capacitance retention of ˜29% from 0.29 F cm²to 0.085 F cm⁻² when increasing the current density from 5 mA cm² to 200mA cm². By contrast, the HACNTs electrode shows much higher arealcapacitance retention of ˜67% from 0.72 F cm⁻² to 0.48 F cm⁻² for thesame current range, consistent with expectations due to the advantagedmicro- and nano-morphologies of the well-aligned nanostructure of HACNTsthat allows the ions to have enough time to transport into the interiorof the electrode even in short time/at high speed (high currentdensity). Hence, the high areal capacitance can be still maintained athigh current density. After coating with P3MT via the oCVD method, theareal capacitance of the composite electrode at 5 mA cm⁻² increasesfurther to as high as 3.11 F cm⁻², which is much higher (from two ordersto several time higher) than that of many other flexible electrodesreported so far. (See, e.g., S. Wang et al., Small, 2017, 13, 1603330;Y. Ko et al., Nature communications, 2017, 8, 536; X. Liu et al. Small,2018, 14, 1702641; Y. Bu et al., Electrochimica Acta, 2018, 271,624-631; S. Zeng et al., Journal of Materials Chemistry A, 2015, 3,23864-23870; H. Moon et al., Scientific reports, 2017, 7, 41981; and N.Wang et al., Journal of Power Sources, 2018, 395, 228-236.) Afterincreasing the current density by 40× to 200 mA cm², the nanocompositeelectrode can still exhibit 1.81 F cm⁻² with capacitance retention morethan 58%. To the best of our knowledge, this is the first work to retainhigh (above 1 F cm²) areal capacitance at such a high (200 mA cm²)current density. The high electrochemical performance of thenanocomposite electrode under high current density is next shown tocontribute to high power density of the assembled cell.

To further explore the potential of the HACNT and P3MT/HACNT electrodes,flexible asymmetric supercapacitor cells were assembled with P3MT/HACNTsnanocomposites as the positive electrode, and HACNTs as the negativeelectrode. FIG. 6A exhibits the sandwich structure of the flexible cellincluding two asymmetric electrodes and a porous separator. The entirecell was encapsulated in polydimethylsiloxane (PDMS) as a flexiblesubstrate and thin copper foils were used as the current collectors (seeExperimental section for details). FIG. 6B presents the CV curves atscan rates from 5 mV s⁻¹ to 200 mV s⁻¹ between 0 V and 3.5 V in atwo-electrode measurement system. All the CV curves exhibitquasi-rectangular and symmetric shapes, indicating an ideal capacitivebehavior and high conductivity due to the horizontal alignment of thenanotubes in the flexible electrodes. To evaluate the capacitiveperformance of the cell, GCD curves at different current densities from5 mA cm⁻² to 500 mA cm⁻² were investigated. As shown in FIG. 6C, thesymmetric and linear charge and discharge characteristics at all currentdensities reveal a rapid I-V response and reversible electrochemicalreactions. The areal capacitances of the flexible asymmetric cell atvarious current densities were then calculated and presented in FIG. 6D.At 5 mA cm⁻², the cell exhibits the areal capacitance of 0.64 F cm⁻²,which is also much higher compared with most state-of-the-art worksbased on carbon, other CP and metal oxide materials. (See, e.g., S. Wanget al., Small, 2017, 13, 1603330; Y. Ko et al., Nature communications,2017, 8, 536; X. Liu et al. Small, 2018, 14, 1702641; Y. Bu et al.,Electrochimica Acta, 2018, 271, 624-631; S. Zeng et al., Journal ofMaterials Chemistry A, 2015, 3, 23864-23870; H. Moon et al., Scientificreports, 2017, 7, 41981; and N. Wang et al., Journal of Power Sources,2018, 395, 228-236.) The capacitance decreased from 0.64 F cm⁻² to 0.32F cm⁻² when the applied current density increased by 100×, showingsuperior capacitance retention performance to other works cited above.The high capacitance, as well as high capacitance retention, can beattributed to several factors: (i) the synergistic effect of highpseudocapacitance for P3MT and high conductivity for HACNTs. The HACNTsprovide a stable mechanical nanostructure for ions transfer in thechannels between CNTs and ion diffusion in the bulk of electrodes. (ii)the provision of interconnected pathways ions through the pore networkof the polymer due to the highly porous structure of the electrode andenhanced ions transport during the charge/discharge process. Theseproperties translate into high relative areal energy and power densitiesacross a wide range of current densities (details can be found in theExperimental section). A Ragone plot describing the relationship betweenareal energy and power density of the flexible asymmetric cell ispresented in FIG. 6E and compared to recent works for comparison. (See,e.g., L. Dong et al., Nano energy 2017, 34, 242; X. Liu et al., Small2018, 14, 1702641; N. Wang et al., Journal of Power Sources 2018, 395,228; S. Wang et al., Small 2017, 13, 1603330; L. Dong et al., AdvancedMaterials 2016, 28, 1675.) The assembled asymmetric cell deliversmaximum energy and power densities of 1.08 mWh cm⁻² and 1.75 W cm⁻²,respectively, which are much higher than many other flexiblesupercapacitor cells reported previously based on either symmetric orasymmetric configurations. (See, e.g., S. Wang et al., Small, 2017, 13,1603330; Y. Ko et al., Nature communications, 2017, 8, 536; X. Liu etal. Small, 2018, 14, 1702641; N. Wang et al., Journal of Power Sources,2018, 395, 228-236; and L. Dong et al., Advanced Materials, 2016, 28,1675-1681.) It is noted that the energy density decreases very littlewith increasing power density, indicating excellent energy densityretention. Such performance is highly desired for portable or wearableelectronics demanding fast-charging technologies.

To evaluate the mechanical robustness of the asymmetric supercapacitorcell, the electrochemical performances under mechanical deformation havebeen characterized and compared with those under the unstrained state,as shown in FIGS. 7A-7D. The deformation was achieved with bending angleof 180° and radius of curvature ˜5 mm. As shown in FIG. 7B, there are noapparent changes in the CV curves under 100 mV s⁻¹ between the loadedand unloaded states, indicating a flexible high-performing device. Thesuperior mechanical reliability was also confirmed by the comparisons ofNyquist plots in the impedance test, with little changes as shown inFIG. 7C. The sharp rise of the imaginary part of Nyquist plots in thelow-frequency range indicates ideal capacitive performance of the cells.By enlarging the Nyquist plots in the high-frequency range, theequivalent series resistance (ESR) can be found at the intersectionbetween the plot and the x-axis). From the inset of FIG. 7D, theflexible cell exhibits the same small ESR of 1.7 Ωcm² under both theloaded and unloaded states. The ESR is smaller or competitive comparedwith those in extant works. (See, e.g., S. Wang et al., Small, 2017, 13,1603330; Y. Zhou et al., Electrochimica Acta, 2013, 111, 608-613; and A.Liu et al., Advanced Materials, 2017, 29, 1606091.) The small ESRrepresents the high conductivity of the flexible electrodes andoriginates from the well-aligned nanostructures, giving rise to the highcapacitance retention and high power density observed above. Thecharge/discharge cycling tests at 100 mA cm⁻² while cycling bending(load-unload cycles) were also carried out and the results are presentedin FIG. 7D. The cell was operated alternately in normal flat and bendingstates for 1000 cycles each and finally recovered to its flat state.After 5000 cycles, the asymmetric cell exhibits a high capacitanceretention of 92%, showing that mechanical bending has a small influenceon the ion transport in the electrodes, which we attribute to the strongand robust support of the aligned carbon nanotube scaffold in bothelectrodes. Hence, the flexible cell exhibits characteristics needed anddesired for portable and wearable power sources.

CONCLUSIONS

P3MT/HACNTs nanocomposite electrodes have been designed and fabricatedfor flexible supercapacitor cells. HACNTs, fabricated from a facilerolling method, provide aligned carbon nanofibers to enhance iontransport in and out of the bulk of the electrode, facilitatingpseudocapacitive ion storage in the bulk of the conformal P3MTconductive polymer (CP). Compared with conventional flexible substrateswith random dispersion of fillers, HACNTs provide superior mechanicalsupport and higher electrochemical performance, relative to a directcomparison here to CNT buckypaper. By conformal coating of new P3MT CPon the HACNTs with our unique oCVD method, the areal capacitanceincreased 3× to more than 3 F cm² at 5 mA cm². The high capacitance isretained even at high current density, which is also attributed to thewell-organized nanomorphology of the electrodes. Asymmetric flexiblesupercapacitor cells based on the HACNT and P3MT/HACNT electrodesexhibited remarkable energy and power densities beyond all otherreported works at 1.08 mWh cm⁻² and 1.75 W cm², respectively.Furthermore, the electrochemical performance changed very little undersevere bending, indicating excellent mechanical cycling performancesuggesting the new electrodes are promising for wearable and portableelectronic applications.

Experimental

Preparation of HACNTs:

Aligned CNTs (A-CNTs) were grown firstly by thermal catalytic chemicalvapor deposition (CVD) on silicon wafers using iron (Fe) on alumina asthe catalyst. These as-grown carbon nanotubes (CNTs) have a highlyaligned structure with approximately 1% (1% CNT by volume and 99% air)volume fraction and densities of 10⁹-10¹⁰ CNTs per cm². The averagediameter of these CNTs is 8 nm (multiwall CNTs with 3-5 shells of walls)and the CNT-CNT spacing (center to center) is approximately 80 nm in theas-grown A-CNT forest. The A-CNTs were densified and reorientedhorizontally to HACNTs using a 10 mm diameter Teflon rod or razor bladealong the horizontal alignment direction as shown in the middle of FIG.3A. The thickness is reduced from ˜2.4 mm to 200 μm after densification.Since the post-growth H₂ anneal step weakens the attachment of the CNTsto the catalyst layer, the CNT film adhered on the substrate can beeasily peeled off. For a comparison, the CNT buckypaper was alsoprepared by vacuum filtration of the MWCNT aqueous dispersion. (See M.Endo et al., Nature, 2005, 433, 476.) As described above, the thicknessof the HACNT film is 200 μm and the CNT Vf is 12%, and for comparison,the buckypaper film has thickness 200 μm and Vf˜15%.

Preparation of P3MT/HACNTs:

Deposition of P3MT CP on HACNTs was performed by using the oCVD process.The HACNTs were held facing the oxidizing agent in a vacuum chamber.Heating the oxidizing agent allowed for its sublimation into the CNTarrays. 3MT was polymerized by oCVD technology. (See K. K. Gleason, CVDPolymers: Fabrication of Organic Surfaces and Devices, John Wiley &Sons, 2015.) 3-MT was purchased from Sigma Aldrich and used withoutfurther processing. The monomer, 3-MT, is heated to 130° C., and isintroduced into a vacuum chamber in vapor phase. At the same time, theoxidant iron (III) chloride, is vaporized after being heated up to 200°C. The monomer molecules and oxidant molecules are then mixed in thevapor phase and adsorbed on the substrate HACNT. Upon adsorption, thereaction between oxidant and monomer take place, leading to thepolymerization of 3-MT. During the oCVD process, the temperature of thesubstrate HACNT is controlled as 40° C., and the pressure of the vacuumchamber is controlled as 150 mTorr. This yields a conformal P3MT coatingof 5 nm on the CNTs in the HACNT nanocomposite film.

Fabrication of Flexible Asymmetric Supercapacitor Cells:

The asymmetric cells were prepared by assembling P3MT/HACNTs as thepositive electrode and HACNTs as the negative electrode, which wereseparated by a porous film (Celgard 3501, Celgard LLC). 1 Mtetraethylammonium tetrafluoroborate/propylene carbonate (Et4NBF4/PC)was used as the electrolyte. Each electrode was transferred onto anelastic polydimethylsiloxane (PDMS) substrate (˜200 μm thickness) bypressing the PDMS film on the electrode. The whole cell was thenconstructed by assembling the two PDMS-supported electrodes and thincopper foils (˜50 μm thickness) were used as the current collectors.

Materials Characterizations:

The nanomorphology of Electrodes was characterized by Ultra plus ZeissSEM and FEI Tecnai G² Spirit TWIN TEM. The Raman spectra were obtainedby Horiba Jobin Yvon HR800 in the Institute for Soldier Nanotechnologiesat MIT.

Electrochemical Characterizations:

CV, GCD, and Impedance tests were performed by using a VersaSTAT 4instrument (Princeton Applied Research). The areal capacitance ofelectrode and cell can be obtained from discharge profile in GCD testbased on the equation belowC=I/(dV/dt)  (1)where I is the discharge current density; t is the time during thedischarge process and V is the potential. The areal energy density andpower density can be calculated by using the following equations

$\begin{matrix}{E = {\frac{1}{2}{C\left( {V_{1}^{2} - V_{2}^{2}} \right)}}} & (2) \\{P = \frac{E}{\Delta t}} & (3)\end{matrix}$where V₁-V₂ is the potential window and Δt is the discharge time. WhenV₂=0, Equation 2 simplifies to the following:E=½CΔV ²  (4)

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that embodiments describedherein are presented by way of example only and that, within the scopeof the appended claims and equivalents thereto, the invention may bepracticed otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A method, comprising: rearranging a plurality ofelongated electronically-conductive nanostructures on a surface suchthat the plurality of elongated electronically-conductive nanostructurestransition from a first arrangement in which longest dimensions of theelongated electronically-conductive nanostructures are orientedsubstantially perpendicular to the surface to a second arrangement inwhich a majority of the elongated electronically-conductivenanostructures have longest dimensions oriented substantially parallelto the surface and oriented more greatly in a first direction parallelto the surface than in another direction perpendicular to the firstdirection; and associating a pseudocapacitive material with theplurality of elongated electronically-conductive nanostructures afterthe rearranging such that: the majority of the elongatedelectronically-conductive nanostructures are conformally coated with thepseudocapacitive material, a thickness of the pseudocapacitive material,over at least 80% of a surface area of the elongatedelectronically-conductive nanostructures that are coated with thepseudocapacitive material, does not deviate from an average thickness ofthe pseudocapacitive material by more than 50%, and the majority of theelongated electronically-conductive nanostructures have a first end thatis attached to the surface and a second end opposite the first end thatis not attached to the surface.
 2. The method of claim 1, wherein therearranging comprises rolling a mechanical device across the pluralityof elongated electronically-conductive nanostructures.
 3. The method ofclaim 1, wherein the associating comprises depositing thepseudocapacitive material via chemical vapor deposition.
 4. The methodof claim 1, wherein the pseudocapacitive material is an electronicallyand/or ionically conductive polymer.
 5. The method of claim 4, whereinthe pseudocapacitive material comprises poly(3-methylthiophene).
 6. Themethod of claim 1, wherein the elongated electronically-conductivenanostructures are carbon based.
 7. The method of claim 1, wherein theelongated electronically-conductive nanostructures comprise carbonnanotubes.
 8. The method of claim 1, wherein the elongatedelectronically-conductive nanostructures comprise multi-walled carbonnanotubes.